Three dimensional (3D) marine seismic surveys are surveys conducted in marine environments, both in the saltwater sea (offshore) and fresh and brackish waters. A typical survey of this kind deploys at least one acoustic source and corresponding sensors at given mutual distances, in a survey arrangement like the one belonging to prior art and illustrated in FIG. 1 of the accompanying drawings. The arrangement comprises a vessel 2, towing lines 18, 20 and connections, and a group or array 6 of trailing survey cables usually called streamers 8 and carrying the acoustic source(s) 4 and sensors 10. Acoustic pressure waves from each source 4, which propagate downwardly through the water into the geological formations, are reflected as shear and pressure waves from the various structures and interfaces of same and may reach the sensors 10 at detectable levels. A subsequent conditioning and processing of the detected waves then convert them to seismic data, to be analysed for the possible indication of locations of hydrocarbon deposits.
A common marine survey is conducted by using such a towed array 6 of parallel streamers 8 that follow fairly straight lines backwards from the vessel 2 stern. Each streamer normally carries a large number of acoustic sensors 10 or receivers, usually called hydrophones. The length of the streamers is today often in the range 600-12 000 m, and their number may be from two to more than twenty.
In conventional seismic towed-array surveys the streamers are commonly held some 50-100 m apart during towing, which has been found convenient with respect to the covered survey area and the compromise between the detection of reflected waves and the discrimination and suppression of linear noise overlaying the detected and processed acoustic signals. The total number of seismic streamers is also constrained by the towing capacity of the vessel.
An alternative to the conventional seismic array is the one called the “wide tow” array in which the maximum value of the streamer spacing, according to the theory of discrete sampling, is exceeded. The corresponding reduced density of the detection signals is in this case compensated by inter/extrapolation, so that a seismic data density fairly close to the level known from the conventional towed-array survey is achieved. In addition one gets the benefit of technical advantages as well as lower costs, partly due to fewer vessel passes for a given survey coverage. A specific such advantage is that the incidence angles of the acoustic wave are allowed to be wider, both at the reflecting formation structures and the streamer hydrophones.
Between the winch for launching and rewinding on board the vessel and the streamers in position in the sea there are provided so called lead-in cables 18. These cables are usually of the triple layer armoured type and designed to withstand the rough conditions often met at the forward end of the streamer array. The cables 18 are adapted for the combined supplying of electric power to the sources and sensors along the streamers, transferring signals and pulling by towing, so as to deploy the streamers from the seismic vessel 2 and maintain the streamers 8 at a selected mutual lateral distance behind it. Further is often used a stretchable rope or streamer section 9 at the forward and in case also the aft end of each streamer 8 for taking up tugging forces in heavy sea.
In order to provide separation of the streamers at both ends, deflection devices 12 setting up opposite lateral forces (“lift” and drag) are used. These devices are called paravanes and act as the well known fishing tackle otter. They are kept at an angle to the towing direction of the array 6, so as to produce a separating (“lateral lift”) force due to the movement in the water. The forward paravanes 12 (only one such is illustrated in FIG. 1) may have a separating (or lift) capacity in the order of 150 kN—equivalent to a weight of 15 tons—and a surface area of up to 40 m2. Each paravane is connected to primarily the outermost one of the lead-in cables 18, usually indirectly through a laterally outermost part 22 of a “super wide” rope 14, 22, also called spreader rope or cable, to which the streamers 8 are fixed near their forward ends. The super wide rope extends transversely to the direction of motion of the vessel 2, thereby forming tethers 14 between each two streamers. When the super wide rope is maintained at a correct tension, these tethers 14 substantially fix the relative lateral positions of the forward ends of the streamers. The paravane is as well connected to a corresponding outermost tow-in rope 20 which, together with the part 22 and their connections to each other and to the paravane 12 form a local rope array or assembly often bearing the name bridle 16 and partly including the wires or ropes 23 fixed to the paravane body. At least one of the connections within said bridle 16 is serving as a force converging point 17 (see FIG. 1). The connections, and in particular the one representing the converging point 17, are normally heavy fixed assemblies that during the survey are held just below the water surface.
Besides, it is naturally that the most suitable configuration for said bridle 16 or rope array that ultimately couples the paravane to the super wide rope 14, 22 and/or the tow-in rope 20 may vary depending on the particular paravane used, and on actual vessel motion conditions.
Prior art is abundantly represented by patent documents, of which a few ones covering streaming arrays and paravane systems are listed below:
GB 2 415 675, our own NO 99 6452, NO 2007 3824—PGS, NO 2008 3173, NO 2009 0530—Geco, U.S. Pat. No. 4,574,723 and U.S. Pat. No. 7,577,060.
The normal or typical survey speed of vessels towing a multiple streamer array of the conventional “narrower” type is today 7.5-10 km/h (4-5 knots), primarily limited by the transverse forces acting on the lead-in cables 18, in particular the outermost ones, as well as the tow ropes 20, often named “spur lines”, as these cables and ropes have the widest angle relative to the vessel heading and towing direction. Secondarily, but as important, is the coupling between said outermost lead-in cables 18 and spur lines 20, and the super wide rope 14, 22 in the transverse direction and connected to the forward end of the streamers 8.
Still more demanding in this respect is the wide tow array having an extended streamer spacing and/or a greater number of streamers and thereby a larger width and corresponding wider lead-in angles, and it is evident that both the paravanes and the bridles then have to be dimensioned accordingly.
FIG. 2 of the drawings illustrate another version of the bridle 16 centred on the force converging point 17, in that a connecting rope 24, often named “lever arm”, is inserted between said converging point and the paravane 12 through its fastening wires or ropes 23. The dimensioning of said lever arm rope 24 has traditionally called for a high strength man-made fibre rope of inter alia the Dyneema® brand, at a length between 3 and 30 m, as this rope is to be regarded as a crucial element for the entire seismic array 6. The working load of—or the tension in—said element is in fact found to occasionally exceed twice the value of that of the spur lines 20 and may reach forces equivalent to 100 tons or more.
In addition the fibre lever arm rope 24 of today can be regarded to be quite vulnerable in the sea, i.e. regarding overstretching, fatigue rupture and damage due to possible contact with parts of fishing lines etc., even more as such contacts more easily lead to failure when such a rope or similar is under great tension.
Moreover are typically used so called soft splices for the lever arm rope 24 connections at both ends, due to that the survey array 6 normally has to be recovered over an overboard sheave. So, even if the lever arm rope—in particular in a wide tow survey array—turns out to be the most severely loaded element in said array, in fact having to withstand forces of the kind and order mentioned above, “soft” and vulnerable components of limited strength and life expectancy are today apparently still the best choice.
The continuous need for improvements to increase the seismic survey efficiency has however led to attempts for using a component suited for more solid connections than said soft splices, as this seemed near at hand for the developing professional. However, this idea was abandoned and did not find practical use due to the apparent greater risk for damaging the winch/sheave mechanism.
On the background of this, the objective of the invention was to find a better overall solution to this problem, namely reducing or eliminating the risk of damaging the sheave and corresponding mechanisms, but at the same time strengthening the bridle components, in order to obtain a much longer and more predictable life time with respect to fatigue and damage by contact, an easier assembly and disassembly, possible lower costs and generally more failure safety.